Process for treating iron-containing waste streams

ABSTRACT

The invention is a process for treating an iron-containing waste stream. The process comprises dividing the waste stream into a first and a second stream, then adding a neutralization agent (e.g., calcium hydroxide) to the first stream to form a partially neutralized slurry, and combining this partially neutralized first stream with the second stream to form a combined stream. The combined stream comprises ferrous chloride and metal hydroxide precipitates, which are separated.

FIELD OF THE INVENTION

This invention relates to a process for treating iron-containing wastestreams produced in the chlorination of titaniferous raw materials.

BACKGROUND OF THE INVENTION

The manufacture of titanium dioxide pigment is commercially performed byeither the sulfate process or the chloride process. The chloride processfirst converts titania-containing feedstocks to titanium tetrachloridevia a high temperature (800-1200° C.) carbochlorination reaction that isperformed in a chlorinator in the presence of chlorine gas and petroleumcoke added as a reductant. Since titania-containing feedstocks, such asores and slags, typically contain many elements in addition to titanium(e.g., Fe, Mn, Cr, V, Al, Nb, Mg, Si, Zr, and Ca), the chlorinationreaction produces other volatile and non-volatile metal chlorides, oroxychlorides, in addition to titanium tetrachloride. The titaniumtetrachloride product is purified by separation from the other metalchlorides or oxychlorides prior to oxidizing the titanium tetrachlorideto form titanium dioxide pigment.

Historically, the chlorinated impurities have been separated from thetitanium tetrachloride and disposed as waste. However, with increasingenvironmental regulation and decreasing availability of landfills, therehas been a movement to find uses for the impurities, as well as todevelop methods to render them useful.

Since iron is a major impurity in titania-containing feedstocks, manymethods have been proposed to utilize the iron by-products. U.S. Pat.No. 4,994,255, for instance, teaches a process wherein the ferrouschloride by-product of chlorination is oxidized to produce iron oxideand chlorine gas which can be recycled back to the chlorinator reactors.U.S. Pat. No. 5,282,977 teaches a neutralization and precipitationprocess for separating chromium, vanadium, and titanium from waste watergenerated by the sulfate or chloride process. The process is taught ashaving a lesser effect on iron.

U.S. Pat. No. 6,800,260 discloses a process for producing iron oxidesfrom the treatment of iron-containing waste streams by a neutralizationand oxidation procedure. One of its disclosed embodiments comprisesfirst dividing a liquid slurry stream into a first and second slurrystream, then adding a calcium-containing neutralization agent to thefirst slurry stream to form a metal hydroxide-containing precipitate anda calcium chloride-containing liquid phase. A majority of the calciumchloride-containing liquid phase is separated from the metalhydroxide-containing precipitate, and the metal hydroxide-containingprecipitate is then added to the second slurry stream to form a firstprecipitate and a first liquid phase, which are then separated. Lastly,the first liquid phase is lastly subjected to an oxidation,neutralization and precipitation process to form an iron-containingcompound.

A process for purifying an acidic technical-grade iron chloride solutionformed in the chloride process is taught in U.S. Pat. No. 5,407,650. Theprocess teaches first adjusting the pH with a first neutralizing agent(e.g., CaCO₃) and thereafter introducing the pH adjusted solution in acontrolled manner into a solution containing a second neutralizingagent. The undesired ions (such as Cr, V, Zr, and/or Nb) precipitate inthe form of hydroxides which can be filtered.

U.S. Pat. No. 5,935,545 discloses a process for preparing an aqueousFeCl₃ solution, comprising the steps of: (a) reacting a titaniferous orewith chlorine and coke to form a metal chloride vapor stream comprisingtitanium tetrachloride, ferrous chloride, ferric chloride and unreactedcoke and ore solids; (b) cooling the metal chloride vapor stream to atemperature in the range of 350 to 500° C. to condense at least some ofthe ferrous chloride; (c) separating the condensed ferrous chloride andthe unreacted coke and ore solids from the metal chloride vapor stream;(d) cooling the metal chloride vapor stream to a temperature in therange of 180 to 240° C. to form a precipitate comprising ferricchloride; and (e) adding the precipitate to water to form an aqueoussolution comprising ferric chloride.

In sum, there remains a need to develop cost-effective processes fortreating chlorination streams that contain iron impurities and toretrieve useable iron containing products from these streams.

SUMMARY OF THE INVENTION

The invention is a process for treating an iron-containing effluentproduced by the chlorination of a titaniferous feedstock. The processcomprises dividing the effluent into a first and a second stream, thenadding a neutralization agent (e.g., calcium hydroxide) to the firststream to form a partially neutralized slurry, having a pH of 4.7 orgreater. The partially neutralized first stream is then combined withthe second stream to form a combined stream comprising ferrous chlorideand metal hydroxide precipitates, and having a pH of 3.2 to 4. Lastly,the ferrous chloride is separated from the metal hydroxide precipitates.Surprisingly, the partial neutralization of a portion of the wastestream and recombination with the remainder of the waste stream resultsin avoiding the formation of a gelatinous mass of precipitatesencountered if the process is performed in a single stage.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a process for treating an iron-containing effluentproduced by the chlorination of a titaniferous feedstock. Titaniferousfeedstocks are raw materials that contain significant amounts oftitanium dioxide and iron oxides, in addition to other impurities.Particularly preferred titaniferous feedstocks include anatase ores,ilmenite deposits, slags, and tar sands. Preferred titaniferousfeedstocks contain about 40 to 80 weight percent titanium dioxide andabout 20 to 50 weight percent iron oxide. They typically also containfrom about 0.04 to 2 weight percent MnO, Cr₂O₃, and V₂O₅.

The iron-containing effluent of the invention is typically produced inthe chloride process. The chloride process is well known in the art.See, for example, U.S. Pat. Nos. 2,486,912 and 2,701,179. In thechloride process, the titaniferous feedstock is chlorinated at hightemperatures (800-1200° C.) in the presence of chlorine gas andpetroleum coke added as a reductant. The reaction is typically performedin a fluid-bed reactor, although static bed reactors may also be used.The chlorination reaction produces a mixed chloride stream thatcomprises titanium tetrachloride and ferrous chloride, in addition toother volatile and non-volatile metal chlorides.

In order to use the titanium tetrachloride in the production of titaniumdioxide pigment, it is necessary to separate the titanium tetrachloridefrom the other metal chlorides. A crude titanium tetrachloride stream isseparated from the mixed chloride stream to leave the iron-containingeffluent. Typically, the mixed chloride stream is cooled (preferably toabout 150-450°) in a cooling vessel, such as a cyclone. Low-volatilemetal chloride impurities (e.g., iron, manganese, magnesium, andchromium) are condensed in the cooling vessel to produce theiron-containing effluent while the crude titanium tetrachloride isseparated as a vapor stream.

Preferably, substantially all of the iron in the iron-containingeffluent will be ferrous chloride (iron (II) chloride). Theconcentration of ferrous chloride in the iron-containing effluent is notcritical, however, preferably the iron-containing effluent will be asconcentrated as possible in terms of the ferrous chloride. Typically,the iron-containing effluent will also contain the chlorides andoxychlorides of other metals. Examples of these chlorides andoxychlorides include, but are not limited to, the chlorides andoxychlorides of titanium, manganese, chromium, vanadium, aluminum,niobium, magnesium, calcium, silicon and zirconium. The titanium in theiron-containing effluent will typically be residual titanium. Aspreviously discussed, most if not all of the titanium will preferablyalready have been removed so that it may be processed separately.

The iron-containing effluent is processed according to the process ofthe invention. The process for treating the iron-containing effluentfirst comprises dividing the iron-containing effluent into two streams:a first stream and a second stream. The first stream preferablycomprises 25 to 45 weight percent of the entire iron-containing effluent(by weight or volume), most preferably 30 to 40 weight percent. Thesecond stream comprises the remainder of the iron-containing effluent.

The first stream is treated with a neutralizing agent. The neutralizingagent is added to the first stream to form a partially neutralized firststream. Preferably, the step of adding the neutralizing agent isaccompanied by stirring or otherwise mixing the neutralizing agent withthe first stream. The pH of the first stream (and therefore theiron-containing effluent) prior to the addition of the neutralizationagent is typically less than pH 2.5, preferably from pH 1.5 to 2.5.Following addition of the neutralizing agent, the partially neutralizedfirst stream has a pH of 4.7 or greater.

Neutralizing agents are any basic substances that are capable of raisingthe pH of the first stream to a pH of 4.7 or greater. Preferably, theneutralizing agent is a calcium-containing substance. Calcium-containingsubstances tend to be relatively inexpensive, though relatively pure,and the cakes that they form, when the precipitates are filtered, arerelatively easily retrieved. The phrase “calcium-containing substance”refers to a substance that contains calcium and that is useful forneutralizing solutions that contain metal chlorides. The neutralizingagent may also be a sodium-containing substance such as sodiumcarbonate. Preferred neutralizing agents include calcium hydroxide,calcium oxide, and mixtures thereof. Calcium hydroxide is mostpreferred. The amount of neutralization agent that one uses will easilybe determined by persons skilled in the art. In part, the amount isdependent on the amount and character of the first stream and theneutralizing agent itself.

The addition of the neutralization agent to the first stream will yielda liquid phase and precipitates of metal hydroxides. The precipitatesare metal hydroxides that are formed in the first stream that arecapable of precipitating when the pH is changed to a pH of 4.7 orgreater, preferably to a pH of 4.7 to 5.3. For example, if theiron-containing effluent contains the chlorides of Al, V, Cr, and/or Nb,the precipitate may contain the hydroxides of these metals.

Following the treatment of the first stream, the partially neutralizedfirst stream is combined with the second stream to form a combinedstream. The terms “combined” and “combining” refer to any methods thatare either now known or come to be known to persons skilled in the artfor introducing substances to be combined with each other. Combining maybe accompanied by stirring or otherwise mixing the substances to becombined. The combined stream comprises ferrous chloride and metalhydroxide precipitates. Following combination of the partiallyneutralized first stream with the second stream, the combined streamwill have a pH of 3.2 to 4, preferably a pH of 3.2 to 3.7.

Lastly, the ferrous chloride is separated from the metal hydroxideprecipitates. Methods for separating a precipitate from the liquid phaseout of which it has been precipitated are well known to persons skilledin the art and by way of example include, but are not limited to,filtration, decantation, and centrifugation. Filtration is particularlypreferred.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

EXAMPLE 1 Single Stage Partial Neutralization EXAMPLE 1A NeutralizationRuns #1

Samples of a slurry waste stream are taken from a chloride TiO₂ process.The stream is the feed to the effluent treatment plant. The slurry wastestream is the overflow from the hydrocyclone that recovers coarse oreand coke from the sluice slurry. The coarse ore and coke are recoveredas the underflow product in order to recycle back to the chlorinator.The slurry waste stream samples contain 10-12 weight percent fine solidsin aqueous suspension with the remainder a solution of specific gravity1.3, containing 32 weight percent metal chlorides. The solution contains263-293 g/L ferrous chloride and other soluble metal chlorides inamounts proportional to that found in the starting slag. The pH of thesample is 2.2.

Portions of the waste stream sample are partially neutralized to pH2.7-4.5 using calcium hydroxide slurry, in order to determine therequired pH value to produce a suitable ferrous chloride liquor assay.In a series of experiments, a waste stream sample is agitated and a pHelectrode is placed in the slurry prior to calcium hydroxide addition.Calcium hydroxide slurry is then added and the resulting slurry agitatedwhile the pH is monitored. When the pH stabilizes, the partiallyneutralized slurry is filtered and the filter cake is washed with water.This combined washings and filtrates are analyzed for metal chloridesconcentration.

A portion of the waste stream sample used in the experiments is alsofiltered, and these filtrates are analyzed for metal chloridesconcentrations. The percentage of each metal chloride precipitated isthen calculated from the analyses of the waste stream sample and thepartially neutralized samples.

The results (see Table 1) show that samples taken to a final pH value of<3 result in formation of gelatinous precipitates during addition of thecalcium hydroxide. This is more pronounced at lower pH values. In aspecific example, an immobile reacting mass containing gelatinousprecipitates is formed at pH 2.7 such that the reacting mass could nolonger be agitated, in order to proceed further with the neutralization.The immobile slurry could not be filtered in a reasonable time, incomparison to filtration times of 9-16 minutes for higher pH values.

It is also found from analysis of the liquor obtained from the partiallyneutralized slurries that the pH required for the partial neutralizationprocess is a minimum of 3.2. This minimum pH ensures that the salts ofAl, Cr, Nb, Si, Ti, V, and Zr (i.e., those salts other than Fe, Mg, Mn &Ca) are precipitated, and can therefore be removed by filtration. Theanalysis shows that the salts of Al, Cr, Nb, Si, Ti, V, and Zr arepredominantly precipitated from solution and do not appear in thefiltrate (see Table 2).

EXAMPLE 1B Neutralization Runs #2

Slurry waste stream samples are taken from a chloride TiO₂ process asdescribed above. The samples contain up of 10 weight % fine solids inaqueous suspension with the remainder a solution (specific gravity 1.33)containing 32% weight percent metal chlorides. The samples contain 270g/L ferrous chloride and other soluble metal chlorides in amountsproportional to that found in the starting slag. The pH of the sample is2.5.

Portions of the starting material are partially neutralized to pH 3.8—pH5.1, in a single stage, using calcium hydroxide slurry according to theprocedure of Example 1. The time required to reach a stable pH value isrecorded in each case (see Table 3). It is found that the reaction timerequired for partial neutralization to pH 5 was 10 minutes compared togreater than 120 minutes for pH less than 4. The relatively longneutralization times required to reach pH 3.2-4 results in the slurryspending a considerable amount of time passing through a pH of less than3. This is the pH zone where gelatinous precipitates form.

These results demonstrate the advantage of taking the slurries to ahigher pH value, with a quicker neutralization, thus avoiding the pHzone where gelatinous precipitates form. However, this pH is outside thewindow to produce a suitable ferrous chloride assay due to losses of Fevalue by precipitation and dilution associated with alkali slurry added.

EXAMPLE 2 Batch Two-Stage Partial Neutralization

A portion of the slurry waste stream sample material from Example 1B istaken to pH 5 and then acidified down to pH of 3.5-4.0, by addition offurther starting slurry waste stream material. The pH is monitored untilthe pH stabilizes.

The results, shown in Table 4, demonstrate that the reaction time for atwo stage partial neutralization is comparable to a single stage partialneutralization. However, the two stage partial neutralization processavoids the pH zone where gelatinous precipitates form.

EXAMPLE 3 Continuous Two Stage Neutralization

Samples of a slurry waste stream are taken from a chloride TiO₂ processover an extended period to provide a feedstock for a continuoustwo-stage partial neutralization. The slurry waste stream samplescontain 10-11 weight percent fine solids in aqueous suspension with theremainder a solution having specific gravity 1.32-1.33, containing 34-35weight percent metal chlorides. The solution contains 306-320 g/Lferrous chloride and other soluble metal chlorides in amountsproportional to that found in the starting slag. The pH of the sample is1.8-2.5.

A continuous two-stage partial neutralization process is run byutilizing two continuous flow stirred tank neutralization reactors, onereactor being the first stage reactor and the other the second stagereactor. The first stage reactor is fed with waste stream slurry andcalcium hydroxide slurry, the flow of calcium hydroxide slurry beingcontrolled to give a first stage partially neutralized slurry pH targetof 4.7 to 5.3. The combined flow of waste stream slurry and calciumhydroxide slurry is controlled to ensure sufficient residence time tocomplete the reaction to pH 4.7 to 5.3. The product, first stagepartially neutralized slurry, is then overflowed into the second stagereactor.

The second stage reactor is fed with first stage partially neutralizedslurry and waste stream slurry. The waste stream slurry flow iscontrolled to achieve a product pH of 3.2 to 4 and ensure sufficientresidence time is allowed for complete reaction. The product, secondstage partially neutralized slurry, from this second stage reactor isthen overflowed to a storage tank for later filtration by a filter pressunit.

Slurry waste stream is continuously fed (17-23 kg/hr) to the first stageneutralization reactor such that 30-40% of the stream is fed to thefirst stage and the remainder 60-70% is fed to the second stage. In thefirst stage, the slurry waste stream is partially neutralized withcalcium hydroxide slurry. The resulting partially neutralized slurryproduct from the first stage is then reacted with the remainder 60-70%slurry waste stream in the second stage. The first stage pH ismaintained at a pH of 4.7-5.3 and the second pH is maintained at a pH of3.4-3.6.

The continuous reactor is run for a total of 243 hours in threecampaigns of 154, 43 and 46 hours. The process is stable, did notproduce any problems with gelatinous precipitates with the whole rangeof starting material concentrations, and produces suitable ferrouschloride liquor. See Table 5.

Samples of product from the continuous unit are collected and filtered,the resulting liquor is analyzed and found to contain the chlorides ofFe, Mn, Mg, Ca with only traces of Al, Si, Ti and Nb. An assay of atypical ferrous chloride liquid product shows 19.7 wt. % FeCl₂, 4.5 wt.% CaCl₂, 3.9 wt. % MnCl₂, 2.6 wt. % MgCl₂, just traces (<0.1 wt. %) ofNbOCl₃ and the chlorides of Al, Si and Ti.

TABLE 1 Results of Single Stage Neutralization pH of Partially 2.7 3.03.3 3.5 4.0 4.5 Neutralised Slurry Filtration Time >180 16 15 10 9.5 9.5(minutes) % FeCl₂ in Filtrate — 20.7 19.4 20.3 17.7 17.7 Total Fe, Mg,Mn, and Ca — 28.9 27.2 26.4 24.3 24.4 chlorides in Filtrate

TABLE 2 Precipitated Metal Results of Single Stage Neutralization pH ofPartially Neu- tralised Percent of Metal Precipitated (%) Slurry Al CrNb Si Ti V Zr 2.5 3.95 12.33 30.16 0 39.72 17.14 37.21 2.7 33.43 21.3139.28 15.23 94.86 86.02 100 3.2 90.05 100 77.8 100 100 100 100 4.3 100100 77.8 100 100 100 100

TABLE 3 Single Stage Partial Neutralization Starting Time to reach finalMaterial: Lime Final pH pH (min) 4.6 5.1 10 11.1 4.4 90 13.1 3.9 12015.0 3.8 180

TABLE 4 Two Stage Partial Neutralization 1^(st) Stage 2^(nd) StageStaring Starting Material: Total Material: Final Time 1^(st) Stage FinalTime Time Lime pH (min) Slurry pH (min) (min) 4.6 5.12 10 1.8 3.99 90100 4.6 5.32 10 2.2 3.71 170 180 5.0 5.20 10 1.8 3.62 210 220

TABLE 5 Continuous Two Stage Partial Neutralization Run 1a 1b 2 3Starting Material % Solids  9.3–13.4 12.8–15.5 19.1 10.4–11.1 % Salts21.9–24.8 20.1–26.8 28.4 34.1–35.9 % FeCl₂ 14.3–16.0 13.1–17.2 18.423.0–24.2 Mg Processed 5.3 4.3  2.3 3.2 Mg to 1^(st) Stage 2.1 1.3  0.81.2 Mg to 2^(nd) Stage 3.2 3.0  1.5 2.0 Partial Neutralization 1^(st)Stage pH 4.5–5.0 4.5–5.8 4.9–5.5 4.7–5.3 2^(nd) Stage pH 3.4–3.9 3.4–4.23.3–3.7 3.4–3.7 Mg Product 5.8 4.7  2.5 3.6 (Slurry) % FeCl₂ 14.7–17.012.8  14.3 19.1–20.0 % Salts 18.1–20.8 19.4  22.6 30.5–31.1

1. A process for treating an iron-containing effluent produced by thechlorination of a titaniferous feedstock, the process comprising: (a)dividing the iron-containing effluent into a first stream and a secondstream; (b) adding a neutralization agent to the first stream to form apartially neutralized first stream having a pH of 4.7 or greater; (c)combining the partially neutralized first stream with the second streamto form a combined stream comprising ferrous chloride and metalhydroxide precipitates, and having a pH of 3.2 to 4; and (d) separatingthe ferrous chloride from the metal hydroxide precipitates.
 2. Theprocess of claim 1 wherein the titaniferous feedstock is selected fromthe group consisting of anatase ores, ilmenite deposits, slags, and tarsands.
 3. The process of claim 1 wherein the first stream contains 25 to45 percent of the iron-containing effluent.
 4. The process of claim 1wherein the first stream contains 30 to 40 percent of theiron-containing effluent.
 5. The process of claim 1 wherein theneutralizing agent is selected from the group consisting of calciumhydroxide, calcium oxide, and mixtures thereof.
 6. The process of claim4 wherein the neutralizing agent is calcium hydroxide.
 7. The process ofclaim 1 wherein the partially neutralized first stream has a pH of 4.7to 5.3.
 8. The process of claim 1 wherein the combined stream has a pHof 3.2 to 3.7.
 9. The process of claim 1 wherein the ferrous chloride isseparated from the metal hydroxide precipitates by a method selectedfrom the group consisting of filtration, decantation, andcentrifugation.